Gas-phase (ion cyclotron resonance spectrometric) and solid-state (crystallographic) studies of highly substituted pyrazoles

The gas-phase basicities of 3,4,5-tri-tert-butylpyrazole (I) and 1,3,4,5-tetra-tert-butyl-pyrazole (II) were measured by Fourier transform ion cyclotron resonance spectrometry. The x-ray molecular structures of I . HCl (a monohydrate) and II were determined, A clear lack of planarity is present in the pyrazole rings because of the steric effects of the tert-butyl substituents. The C(sp)3 atom bonded to N-l, C-3, C-4 and C-5 atoms deviates significantly from the pyrazole plane, as expected on the basis of semi-empirical AM1 calculations, In I . HCl, the molecules form dimers through symmetry centres in which the chlorine atom and the water molecules play an important role.